1. Field of the Invention
The present invention relates to a constant voltage power supply circuit adapted to output a constant voltage.
2. Description of the Related Art
One known constant voltage power supply circuit includes a series linear regulator including a CMOS circuit (see, for example, JPH 2007-219856). The series linear regulator contains a reference voltage generation circuit that generates a reference voltage, a comparator that compares the reference voltage and the output voltage, and a pMOS transistor driven by the comparator.
The pMOS transistor is connected between the input terminal and the output terminal. The input terminal receives an input voltage Vin (i.e., the power supply voltage VDD). The output terminal provides an output voltage VOUT. The voltage VOUT is obtained by decreasing and stabilizing the input voltage Vin.
If the output voltage VOUT decreases due to an increased output load, the input voltage at the non-inverting input terminal of the comparator decreases, thus decreasing the output voltage of the comparator. The gate voltage of the pMOS transistor thus decreases and the ON-resistance of the pMOS transistor decreases. This increases the current supplied to the output terminal, thus increasing the output voltage VOUT. In this way, the voltage VOUT is stabilized.
If the output voltage VOUT increases due to a decreased output load, the input voltage at the non-inverting input terminal of the comparator increases, thus increasing the output potential of the comparator. The gate voltage of the pMOS transistor thus increases and the ON-resistance of the p type MOS transistor increases.
This decreases the current supplied to the output terminal, thus decreasing the output voltage VOUT. In this way, the voltage VOUT is stabilized.
A voltage drop in the output transistor must be small (up to 100 mV) even if a large current (a few hundred mA, for example) flows therethrough. Accordingly, an output transistor with a large gate width is often used therefor. This causes a tendency towards larger gate capacitance. The comparator may be designed to be able to drive the gate capacitance. A rapid change of the output current may, however, cause a driving delay that may vary the output voltage. Accordingly, a need exists for a voltage regulator that may reduce the output variation without complicating the circuitry such as the comparator or increasing the circuitry area.